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Behaviour of passivating layer on potassium electrode in propylene carbonate
BEHAVIOUR OF PASSIVATING LAYER ON POTASSIUM ELECTRODE IN PROPYLENE CARBONATE F. P. DOUSEK and J. JANSTA J. Heyrovskg Institute of Physical Chemistry and Electrochemistry, Czechoslovak lJtov&ren 254, 102 00 Prague 10, Czechoslovakia (Received Abstract-The
electrochemical
23 November
Academy of Sciences,
1983)
behaviour of potassium in highly dried electrolyte (propylene carbonate-
KPF,, -z 0.01 ppm H20) was studied under exclusion of contamination from the atmosphere and changes of the electrolyte concentration during current passage. Polarization curves and long-term dependences of the stationary electrode potential on time showed the existence of a passivating corrosion current, which is suppressed as soon as the electrode is either fully passivated or the solvent molecules at the electrode surface are exhausted by the formation of solvated potassium ions duringanodic polarization. Structural changes of the uassivatinp. laver are accomuanied bv changes of the K+ ions activity in it caunng differences in the eqtihbrium pote&al of the po&ssium electrode.
sequence Li-Na-K (probably owing to increasing atomic radii[ 11, 121, a slower process being easier to follow). The first two properties, (a) and (b), are favourable for purifying the metal, preparation of an electrode from it, and its long-term study in evacuated sealed glass apparatus. Therefore, the present work is devoted to the study of the passivating layer and its changes during polarization on potassium in KPF,-propylene carbonate electrolyte.
INTRODUCTION It is known that for thermodynamic reasons the alkali metals are unstable with respect to any aprotic solvent used in galvanic cells[l] and their use as electrodes is made possible only by the existence of a passivating layer formed by chemical reactions of the metal with the electrolyte components[2,3]. Most of the research work in the field since 1970 has been devoted to lithium. This is because lithium has the highest energy density (Table 1). In spite of great effort, the knowledge of the passivating layer is insufficient to enable the development of a rechargeable alkali metal electrode. Several models were proposed[36]. but more research is needed since the cycle life of lithium accumulators is as yet too short[3, 7, S]. Similar studies on potassium arerareC9Jalthough it is probable that the mechanism of passivation is similar for all alkali metals[lO] (disregarding the quality of the passivating compound), since the solvent instability is due to the extremely negative electrode potential. The difference in standard potentials of Li and K is only 0.1 V, so that the study of the latter could be of significance for the Li electrode as well. The advantage of potassium over lithium for research purposes is that: (a) it has a low melting point, (b) it does not react with glass apparatus when melted, (c) its amalgam is practically inert towards organic electrolytes and gives an ideal reference electrode[ 1 l] and (d) the corrosion rate decreases markedly in the Table 1. Amount of alkali metal corresponding density of 1 C cm- * Metal Li Na K Rb CS
Thickness (qC-‘cmz) 1.357 2.456 4.712 5.790 7.249
% 100 181 347 427 534
Pure potassium (Merck, GFR) containing 0.6 % Na, 3 x 1O-4 “/, Li, 0.1% Fe, 0.001 “/, Pb and 0.01% Cl-, was melted and filtered in vacuum (0.01 Pa) through a glass capillary into stock ampoules which were sealed. A solution of 0.5 M KPFB in propylene carbonate (PC) (Schuchard, Switzerland, or British Drug Houses) was dried in several steps on a molecular sieve of type 3 A and then 14 days in contact with a liquid eutectic K-Na alloy (65: 35 parts by weight)[13, 141. It contained less than 0.01 ppm H,O (for the determination see ref.[lS]) and 5 x lo-‘% potassium propylene glycolate. Its light yellow colour was probably caused by the presence of some decomposition products of PC containing traces of water in contact with the alloy. The dried electrolyte was filtered in vacuum in an all-glass apparatus into stock ampoules which were sealed and some of them were subjected to analysis. Potassium amalgam containing 1.129 atm % K was similarly filled into sealed glass ampoules. The vacuum used was obtained by repeated evacuation of the apparatus at elevated temperature (3OO”C, if possible) and flushing with helium stored for hundreds of hours at 10 MPa in contact with a pool of the K-Na alloy. Thus, the remainder of the gas at a final pressure of 0.01 Pa (ie about 10” atoms per cm3) consists mainly of He atoms. The measuring cell is shown in Fig. 1; its space (4) was filled in vacuum with the amalgam and sealed. A less than
to a charge
Mass (mgC-‘I 0.0719 0.2383 0.4052 0.8859 1.3774
EXPERIMENTAL
“/: 100 331 564 1232 1916 767
F. P.
768
DOUSEK AND .J. IANSTA
eliminated in order to throw more light on experimental results. It is a great advantage of the apparatus employed (Fig. 1) that the cell once filled can be studied for a long time without additional contamination from the atmosphere. Thus, any change of the measured values could be interpreted as a change in electrochemical properties of the potassium electrode. The auxiliary mercury electrode of 10 cm3 volume and 3 cm’ surface area (in which potassium was gradually accumulated during measurement) ensured constant electrolyte composition up to about 200 mAh of charge passed, a necessary condition for following the equilibrium potential E,, which plays an important role in our work.
Primary stationary passivating layer Fig. 1. Measuring cell. (1) Electrolyte; (2) breakable spherical seal; (3) mercury auxiliary electrode; (4) KHg, reference electrode; (5) sealed connections to stock ampoules; (6) transitorily sealed tip of Luggin capillary; (7) potassium electrode; (8) ‘Kovar” contact. Luggin capillary was sealed at (6) with a very thin glass membrane. Space (7) was filled in vacuum with melted potassium from a stock ampoule; after cooling its surface of 0.82cm2 area was flooded with the electrolyte from a stock ampoule under vacuum. Space (3), which was separated from (1) by breakable spherical seal (2), was filled with degas&d mercury from an evacuated stock ampoule. Seals (2) and (6) were necessary to prevent contamination of the potassium surface with mercury vapour whilst filling the cell, which would considerably influence the properties of the passivating layer. They were crushed only after covering the potassium surface with the electrolyte. Since the tip of the Luggin capillary could not be placed exactly at the potassium surface, which was lowered during long-termanodic polarization by up to several millimetres, we measured the Ohmic potential drop iR to correct the measured potential values. The electrodes were polarized galvanostatically and the potential against the potassium amalgam electrode was recorded by a pen recorder with an input resistance of 10” Ohms. The temperature of measurement was 25°C. Following symbols for potassium electrode potential were used: E,, equilibrium potential; E,, opencircuit potential, potential observed after switching off the polarizing current (after sufficiently long time E, reaches E, value); E, potential of a polarized electrode. RESULTS
The absence of gas evolution showed that the potassium surface was rapidly passivated by a reaction with the electrolyte, probably PC + 2K + CH+ZH
= CH2 + l&CO,.
(1)
Traces of propylene may dissolve in PC without being observable. The passivating layer thus formed will be denoted as primary stationary passivating layer. During preparation of the electrode, when its surface was in contact with traces of reactive gases in vacuum, another layer could be formed that will be denoted. as gas-phase passivating layer. Partial passivation by previous contact with the gas phase was estimated as follows. The equilibrium potential E, was established between - 1.039 and - 1.047 V with fresh electrodes. The calculated potential difference between potassium and the potassium amaIgam used is - 1.045 V (see Appendix). When immediately after wetting the electrode with the electrolyte the potential was less negative, it changed, eg during 30 min from - 1.039 to -1.044V and then remained rather stable. On the other hand, when the initial potential value was more negative, it was relatively stable for tens of hours (E, = - 1.047 V, Fig. 2). Thus, the initial potential was
AND DISCUSSION
When the electrode came in contact with the potassium surface, no gas appeared (although it would be easily observable under the given conditions). Law et ar.[9] however, observed gas evolution in contact with KAlC14-PC electrolyte. We were unable to observe gas evolution in the course of 12,000 h even during periodic anodic polarization at 3 mAcmm2. Since corrosion of alkali metals in organic electrolytes is doubtless a complex effect of many factors, at least those introduced by experimental technique must be
I 0
I
1
2
3
4
I
I
5
6
I
t [ 10’Hrsl
Fig. 2. Change of potential with time after switching off the polarizing current on potassium electrode immersed in electrolyte prior to polarization for 200 h.
769
Potassium electrode
apparently characteristic for the primary stationary passivating layer. The more negative potential indicated probably a lower contamination of the electrode by the residual gases from the vacuum, an effect which cannot be fully excluded. The open-circuit potential (E,) values in the indicated range, or more positive, were also obtained with electrodes after switching off their anodic polarization. This can be attributed to partial depassivation caused by a transitory disturbance of the primary passivating layer giving rise to a corrosion current, by which the electrode was polarized ancdically (see below). The behaviour of electrodes which were for various time intervals in contact with the electrolyte prior to anodic polarization shows the stability of the primary stationary passivating layer (modified gas-phase passivating layer) under these conditions. When the electrode which was at a potential of - 1.047 V for 200 h was polarized for the first time with acurrent density of 1 mAcmS for 6 h, after switching off the current it attained a somewhat more negative E, (Fig. 2). This could be repeated until the value of E, = - 1.061 V was attained. Subsequent 1 h cathodic polarization at the same current density* showed that the latter value is close to the limit corresponding to a most strongly passivated surface in the given electrolyte when the corrosion current drops almost to zero. Indeed, the value of E, = - 1.060 5 0.001 V has been maintained for 12,000 h (the experiment continues at present). The possibility of a durable attainment of a stable E, value from - 1.039 to - 1.061 V is related to changes in the properties of the passivating layer causing probable changes in the activity of K+ ions at the metal surface. Thus, a 20 mV potential increase may be attributed to a decrease in the activity by a factor of 0.46 (see Appendix). Secondnry stationary
passimting
1
0
5
45
I\._-----------2
-1.05 t
a
20
40
60
80
100 t
120
[secl
Fig. 4. Potential change of potassium electrode with time (section I in Fig. 3). (1) After passing a 0.1 mAh charge: (2) after passing a lOmAh charge, both at an anodic current of 1 mAcm_‘.
-i
l.03
x 9 2 w -1.04 -
-I,05 -
L 0
* We assumed that freshly deposited potassium in contact with the electrolyte is most likely to be passivated.
40 t hnl
Fig. 3. Change of potentia1 with time for a potassium electrode immediately after immersion into electrolyte (see text).
layer
The above results are related to a long-lasting effect of the eIectrolyte prior to the first anodic polarization. The behaviour of the potassium electrode during anoclic polarization after only an hour of contact with the electrolyte is shown in Fig. 3 (curve 1). Its E, was -1.047 V. III the first 100s break between 300s polarization cycles (section I) the course of E, as function of time is shown by curve 1 in Fig. 4. During a 1500 s pause (Fig. 3, section II) the value of E, as a function of time passed reproducibly through a minimum, indicating continued spontaneous depassivation (Fig. 5, curve 1). The rise following after the decrease of the potential caused by a new gradual passivation corresponds apparently to a corrosion current proceeding on open circuit. The described behaviour changed substantially if a charge higher than 10 mAhcmm2 (at 1 mAcm_‘) passed through the electrode. Then, after 64 h idling period E, was close to - 1.058 V and the electrode could be easier depassivated (curve 2 in Fig. 3) or passivated (compare the pauses, Figs 4 and 5, curve 2), the course of the E, value being quite different. The experiments with both groups of electrodes
I
1
Iq 35
IO
500
10X
t [secl
1500
Fig. 5. Potential change of potassium electrode with time (section II in Fig. 3). Same conditions as in Fig. 4.
F. P. DOUSEK AND J. JANSTA
770
suggest that the passivating layer is not stationary, its equilibration being very slow. The corrosion current appearing after anodic dissolution of the first portion of potassium under the passivating layer (Fig. 5, curve 1) is probably related to a structural reorganization of the primary stationary passivating layer, where the passivating products formed primarily on the potassium surface contacting the gas phase influence the electrode potential. This reorganization (or perhaps recrystallization) causes a decrease in the surface coverage with the passivating products and the metal surface set free is covered with the secondary stationary passivating layer of different properties as a result of the corrosion current. A similar effect was observed on a lithium electrode in the medium of thionyl chloride[ lo]. Kinetic passivating
layer
There is no reason to believe that the passivating layer on the potassium electrode must be nonporous. For the disappearance of the corrosion current, it is only necessary that the active potassium surface and the large organic solvent molecules be mutually separated. It seems that the ratio between the free and covered metal surfaces determines the value of the exchange current, which is known to be much lower than expected in the case of such a reactive metal. The surface coverage decreases during anodic polarization (compare E-r curves in Fig. 6). After switching on a constant current, the potential decreases abruptly from E, = - 1.060 V to a value by about 0.7 V less negative, evidence for a strong passivation of the electrode in stationary state. From that moment the structure of the passivating layer starts to change, the electrode is gradually depassivated and its potential shifts to more negative values. The equilibration takes more time the lower is the polarization current: 7-10 h at 1 mAcme2, 80 min at 2mAcme2 and 35 min at 3 mAcm- I. An anodic charge of 1 mAh cm - ’ of electrode surface dissolves a 17 pm thick layer of potassium (nearly 11 monolayers per second), which
shows that the original structure of the primary stationary passivating layer undergoes a substantial change from the very beginning of polarization, and it is notable that the establishment of the kinetic passivating layer structure takes several hours. This may possibly be attributed to recrystallization of the passivating products during intense mass transport. Mechanism
of passivation
Our hypothesis about kinetic changes of the passivation layer is based on the following observations. (1) Relatively high iR-free electrode polarization even after attainment of equilibrium during anodic dissolution shows that a passivating layer always exists on the electrode surface at any current density (the surface coverage is considerable and not constant). At defects in the kinetic passivating layer, where the potassium surface is momentarily free, the actual current density is obviously much higher than that referred to the geometric surface area. (2) The current efi%iency of potassium dissolution is close to 100°~. Even when a 3 mm thick layer of potassium was dissolved, the passivating layer did not grow thicker and no sludge formation was observed. (3) The passivating layer adheres firmly to the potassium surface, suggesting the existence of strong adsorption forces. It is known that dissolution of crystalline metals does not proceed uniformly over the whole surface, but preferentially on certain active sites, whose location changes quickly during the dissolution. It seems that the passivation product maintains its contact with the metal surface even during its dissolution, especially in the situation described below at (4). (4) Matsuda et a!.[161 determined the solvation number of K+ ions in PC as N, = 1.3 (in the case of Li+, N, = 3.2). Hence, K ’ ions transferred from the anode during anodic dissolution take off from the active sites (defects in the kinetic passivating layer) a considerable quantity of the corrosion medium, PC. The higher the polarizing current, the less PC molecules are available for the corrosion reaction
2
t [Hrsl
3
Fig. 6. E-S curves for anodically polarizedpotassium electrode. (1) i = 1 mA cm-‘; (2) i = 2 mA cm-‘;
= 3 mAcm_‘.
(3) i
Potassium electrode (1) at the active sites of the electrode surface (at the bottom of the defects in the kinetic passivating layer). Thus, at a higher polarizing current a higher fraction of the electrode surface can persist in the active state. Scarr[l] arrived at a similar conclusion in the case of anodic polarization of a lithium electrode. When the equilibrium surface coverage with the passivating layer is reached, the true current density on the free portion of the potassium surface may be almost independent of the overall current density, and probably this is the reason why electrodes loaded for an hour with different currents attained almost the same potential (Fig. 6). As soon as the polarizing current is switched off, the transport of PC molecules from the metal surface stops and their concentration is restored by diffusion, the corrosion reaction (1) starts, and the value of E, corresponding to a newly formed secondary stationary passivating layer is immediately established (Fig. 5, curve 2). As in the case of establishing the stationary potential of a polarized electrode, the course of the E, value with time depends on the preceding polarizing current density, ie the degree of disturbance of the starting primary stationary passivating layer, as follows from Table 2. CONCLUSIONS The surface of an even potassium
electrode
most carefully prepared is always covered with a gas-phase
passivating layer due to reaction with atmosphere components. This is supplemented with products of corrosion reactions between the metal and the electrolyte to Form a primary stationary passivating layer. The coverage of the potassium surface with the latter need not be complete but, anyhow, further PC molecules cannot get into contact with remaining active sites on the metal. The equilibrium electrode potential attains values from - I.040 to - 1.06i V against a KHgx electrode, according to the degree of passivation, its less negative values indicating some influence of the gas-phase passivating layer on the primary stationary passivating layer. On a polarized anode surface, the latter changes to a kinetic passivating layer, distinguished by a lower coverage of the metal surface dependent on the current density. If the anodic polarization lasts for a sufficiently long time, the temporarily uncovered parts of the active surface of potassium are blocked by corrosion with the electrolyte immediately after switching ofF the current to form a secondary stationary passivating layer, which is no more influenced by the gas-phase passivating Iayer. The components of all passivating layers adhere Table 2. Influence of preceding anodic polarization on the rate of potential equilibration of potassium electrode in KPF,-PC electrolyte at 25°C 2 h anodic
current
(~AcII-~) 1 2 3
Open-circuit potential (in V) after t h of current interruption Z=l t+0 t = 4x -1.048 - 1.042 - 1.038
- 1.053 - 1.048 - 1.044
- 1.0615 - 1.0605 - 1.0600
771
strongly to the potassium surface (probably by chemisorption) and therefore they do not lose their contact with the metal during its anodic dissolution. This is obviously why the passivating layer thickness does not visibly increase even on passing a charge of 0.18 Ahcmm2. Acknowledgements-The authors are indebted to Dr. K. Micka and Prof. J. Koryta of this Institute for helpful discussions and stimulating interest.
REFERENCES 1, R, F. Starr, J. elecrrorhem SM. 117, 295 (1970). 2. R. Jasinski and S. Carroll, J. electrochem. Sot. 117, 218 (1970). 3. B. Serosati, Electrovhim. Acra 26, 1559 (1981). 4. S. P. S. Yen, D. Shen and R. B. Someano, Meeting of the Electrochemical Society, Hollywood, Florida 1980. Extended Abstracts, p. 99. The Electrochemical Society. Pennington (1980). 5. F. Schwager, Y. Cetonov and R. H. Muller, ibid., p. 102. (1980). 6. E. Peled, J. Power Sources 9, 253 (1983). 7. F. W. Dampier, J. ebrlrochem. Sor.128, 2501 (1981). 8. K. M. Abraham, J. L. Goldman and M. D. Dempsey, J. &clrochem Sot. 128, 2493 (1981). 9. H. H. Law. R. Atanasoski and Ch. W. Tobias, Meeting of the Electrochemical Society, Hollywood, Florida 1980. Extended Abstracts, p. 106. The Electrochemical Society, Pennington (1980). 10. N. A. Fleischer and R. J. Ekern, J. Power Sources 10. 179 (1983). 11. F. P. Dousek, J. Jansta and J. &“a, J. eleczroannl. Chem. 46_ 281 (19731. 12. F. &. Doisek &d J. Jansta, Electrochim. Acta 20, I (1975). 13. J.Jansta, F. P. Dousek and J. &a, J. electrooncd. Chem. 38, 445 (1972). 14. F. P. Dousek, J. Jansta and J. l%%a, Chemick6 Listy (in Czech.) 67, 427 (1973). 15. 1. Jansta, F. P. Dousek and J. iiha, .I. elettroun~1. Ch.zm. 44, 263 (1973). 16. Y. Matsuda, H. Nakashima, M. Morita and Y. Takasu, J. electrochem. Sot. 128, 2552 (1981). 17. M. H. Armbruster and J. L. Crenshaw, J. Am. ckem. Sot. 56, 2525 (1934). 18. R. Jasinski. High Energy Batteries. Plenum Press, New York (1967). 19. A. J. de Bethune. The Encvciooedia of Elec’trochemistrv (Edited by C. A. ‘Hampel).‘R&nhold,“New York (1964j. 20. G. M. Giordano, P. Longhi, T. Mussini and S. Rondinini, J. Chem. Thermodyn. 9.997 (1977). 21. G. N. Lewis and F. G. Keyes, J. Am. them. SDC. 34, 119 (1912).
APPENDIX Calculation of the potenliol of potassium agains& potassium amnigam reference electrode We shall consider the galvanic cell KHg,(l)IKPF,(0.5
M)(PC)]PLIK(s).
(Al)
aZ,K+ aI.K+ %z The electrode on the right-hand side is a reversible electrode of the first kind, where the solid potassium surface is covered with a polymolecukr passivating layer (PL) functioning as a solid electrolyte. We assume that the activity of K+ ions in it, n2,K-, is different from that in the cell electrolyte.0.5 M KPF,
772
F. P. Dousm
in PC, where it is equal to 0, K+. The potential hand electrode can be expre&d as EKtlK = ERtjK
of the right-
RT + ~ In a2 K‘ F ’
where Eg+/K denotes standard potential of potassium electrode in the given medium. The left-hand electrode of the cell is a liquid amalgam electrode of the first kind containing 1.129 atm y0 K in contact with 0.5MKPF, in PC. Such an electrode is not covered by any passivating layer[l l-151. Its potential can be expressed as EKH~,,u*
=
E&,/K
RT ul K* +f In --!-F UK
(A3)
AND
J. JANSTA
diffusion potential, however we have no quantitative idea in this respect. Equation (A4) requires the knowledge of standard potentials in the corresponding media, the difference from those in aqueous solution is given by the difference of solvation energies of K+ ions[lS], however no data in this respect are available. If the passivation layer were nonexistent, then Q[,~+ would beequal to u~,~+ and the last term in Equation (A4) would he zero. Then, we could use the known standard potentials in aqueous medium, since their shift by the salvation energy of K* ions would be eliminated by their difference occurring in Equation (A4). The values in aqueous medium are[19,20] EkejK = -22.925V, This would
where E&s ,K+ denotes its standard potential in the given medium ani c1K is the activity of potassium, which was determined as 0.02419 by linear interpolation of tabulated data[17] in mol fraction units, the (rational) activity coefficient being equal to 2.14. The electrical potential difference E of the cell considered corresponds to two reversible processes, namely amalgamation of potassium and transfer of K’ ions from the passivating layer (activity 02) into the electrolyte (activity a, ), or vice versa. By substracting Equation (A3) from (AZ) we obtain RT
(A4) Here, the first three terms on the right-hand side correspond to the free energy of amalgamalion and the fourth refers to the osmotic work due to a difference in the activity of K+ ions between the passivating layer and the electrolyte. The interface between these two media gives rise to the formation of a
Eo,,,x,K+ = --1.975V
at 25°C.
give
E = -2.925
+ 1.975 + 0.05916 logo.02419
= - 1.0456 v, which agrees very well with the value measured by Lewis and Keyes[21], - 1.0481 V, for a potassium-potassium amalgam (1.12665 atm % K) cell in ethyl amine-K1 electrolyte. (The potential of their amalgam differs as little as 5 x lo- ’ V from ours.) They stated that no reaction between ethyl amine and potassium took place, hence no passivating layer was formed. The equilibrium potential values for the cell (Al) measured by us were in the range from - 1.039 to - I .06O V. The latter value was reproducible to within + 1 mV as the limiting value corresponding apparently to a steady structure of the secondary stationary passivating layer formed in each experiment after a sufficiently long time. The difference relative to the calculated value (- 1.045 V) is obviously due to the influence of the passivating layer (change of the standard potential and the activity of Kf ions in this layer). Thus, a decrease in the activity of K+ ions in the passivating layer by 56% would cause a 15 mV increase of the K electrode potential, which would account for the difference. Structural changes of the passivating layer are apparently accompamed by changes of the equilibrium potential of the potassium electrode (Fig. 2).
electrochemical
23 November
Academy of Sciences,
1983)
behaviour of potassium in highly dried electrolyte (propylene carbonate-
KPF,, -z 0.01 ppm H20) was studied under exclusion of contamination from the atmosphere and changes of the electrolyte concentration during current passage. Polarization curves and long-term dependences of the stationary electrode potential on time showed the existence of a passivating corrosion current, which is suppressed as soon as the electrode is either fully passivated or the solvent molecules at the electrode surface are exhausted by the formation of solvated potassium ions duringanodic polarization. Structural changes of the uassivatinp. laver are accomuanied bv changes of the K+ ions activity in it caunng differences in the eqtihbrium pote&al of the po&ssium electrode.
sequence Li-Na-K (probably owing to increasing atomic radii[ 11, 121, a slower process being easier to follow). The first two properties, (a) and (b), are favourable for purifying the metal, preparation of an electrode from it, and its long-term study in evacuated sealed glass apparatus. Therefore, the present work is devoted to the study of the passivating layer and its changes during polarization on potassium in KPF,-propylene carbonate electrolyte.
INTRODUCTION It is known that for thermodynamic reasons the alkali metals are unstable with respect to any aprotic solvent used in galvanic cells[l] and their use as electrodes is made possible only by the existence of a passivating layer formed by chemical reactions of the metal with the electrolyte components[2,3]. Most of the research work in the field since 1970 has been devoted to lithium. This is because lithium has the highest energy density (Table 1). In spite of great effort, the knowledge of the passivating layer is insufficient to enable the development of a rechargeable alkali metal electrode. Several models were proposed[36]. but more research is needed since the cycle life of lithium accumulators is as yet too short[3, 7, S]. Similar studies on potassium arerareC9Jalthough it is probable that the mechanism of passivation is similar for all alkali metals[lO] (disregarding the quality of the passivating compound), since the solvent instability is due to the extremely negative electrode potential. The difference in standard potentials of Li and K is only 0.1 V, so that the study of the latter could be of significance for the Li electrode as well. The advantage of potassium over lithium for research purposes is that: (a) it has a low melting point, (b) it does not react with glass apparatus when melted, (c) its amalgam is practically inert towards organic electrolytes and gives an ideal reference electrode[ 1 l] and (d) the corrosion rate decreases markedly in the Table 1. Amount of alkali metal corresponding density of 1 C cm- * Metal Li Na K Rb CS
Thickness (qC-‘cmz) 1.357 2.456 4.712 5.790 7.249
% 100 181 347 427 534
Pure potassium (Merck, GFR) containing 0.6 % Na, 3 x 1O-4 “/, Li, 0.1% Fe, 0.001 “/, Pb and 0.01% Cl-, was melted and filtered in vacuum (0.01 Pa) through a glass capillary into stock ampoules which were sealed. A solution of 0.5 M KPFB in propylene carbonate (PC) (Schuchard, Switzerland, or British Drug Houses) was dried in several steps on a molecular sieve of type 3 A and then 14 days in contact with a liquid eutectic K-Na alloy (65: 35 parts by weight)[13, 141. It contained less than 0.01 ppm H,O (for the determination see ref.[lS]) and 5 x lo-‘% potassium propylene glycolate. Its light yellow colour was probably caused by the presence of some decomposition products of PC containing traces of water in contact with the alloy. The dried electrolyte was filtered in vacuum in an all-glass apparatus into stock ampoules which were sealed and some of them were subjected to analysis. Potassium amalgam containing 1.129 atm % K was similarly filled into sealed glass ampoules. The vacuum used was obtained by repeated evacuation of the apparatus at elevated temperature (3OO”C, if possible) and flushing with helium stored for hundreds of hours at 10 MPa in contact with a pool of the K-Na alloy. Thus, the remainder of the gas at a final pressure of 0.01 Pa (ie about 10” atoms per cm3) consists mainly of He atoms. The measuring cell is shown in Fig. 1; its space (4) was filled in vacuum with the amalgam and sealed. A less than
to a charge
Mass (mgC-‘I 0.0719 0.2383 0.4052 0.8859 1.3774
EXPERIMENTAL
“/: 100 331 564 1232 1916 767
F. P.
768
DOUSEK AND .J. IANSTA
eliminated in order to throw more light on experimental results. It is a great advantage of the apparatus employed (Fig. 1) that the cell once filled can be studied for a long time without additional contamination from the atmosphere. Thus, any change of the measured values could be interpreted as a change in electrochemical properties of the potassium electrode. The auxiliary mercury electrode of 10 cm3 volume and 3 cm’ surface area (in which potassium was gradually accumulated during measurement) ensured constant electrolyte composition up to about 200 mAh of charge passed, a necessary condition for following the equilibrium potential E,, which plays an important role in our work.
Primary stationary passivating layer Fig. 1. Measuring cell. (1) Electrolyte; (2) breakable spherical seal; (3) mercury auxiliary electrode; (4) KHg, reference electrode; (5) sealed connections to stock ampoules; (6) transitorily sealed tip of Luggin capillary; (7) potassium electrode; (8) ‘Kovar” contact. Luggin capillary was sealed at (6) with a very thin glass membrane. Space (7) was filled in vacuum with melted potassium from a stock ampoule; after cooling its surface of 0.82cm2 area was flooded with the electrolyte from a stock ampoule under vacuum. Space (3), which was separated from (1) by breakable spherical seal (2), was filled with degas&d mercury from an evacuated stock ampoule. Seals (2) and (6) were necessary to prevent contamination of the potassium surface with mercury vapour whilst filling the cell, which would considerably influence the properties of the passivating layer. They were crushed only after covering the potassium surface with the electrolyte. Since the tip of the Luggin capillary could not be placed exactly at the potassium surface, which was lowered during long-termanodic polarization by up to several millimetres, we measured the Ohmic potential drop iR to correct the measured potential values. The electrodes were polarized galvanostatically and the potential against the potassium amalgam electrode was recorded by a pen recorder with an input resistance of 10” Ohms. The temperature of measurement was 25°C. Following symbols for potassium electrode potential were used: E,, equilibrium potential; E,, opencircuit potential, potential observed after switching off the polarizing current (after sufficiently long time E, reaches E, value); E, potential of a polarized electrode. RESULTS
The absence of gas evolution showed that the potassium surface was rapidly passivated by a reaction with the electrolyte, probably PC + 2K + CH+ZH
= CH2 + l&CO,.
(1)
Traces of propylene may dissolve in PC without being observable. The passivating layer thus formed will be denoted as primary stationary passivating layer. During preparation of the electrode, when its surface was in contact with traces of reactive gases in vacuum, another layer could be formed that will be denoted. as gas-phase passivating layer. Partial passivation by previous contact with the gas phase was estimated as follows. The equilibrium potential E, was established between - 1.039 and - 1.047 V with fresh electrodes. The calculated potential difference between potassium and the potassium amaIgam used is - 1.045 V (see Appendix). When immediately after wetting the electrode with the electrolyte the potential was less negative, it changed, eg during 30 min from - 1.039 to -1.044V and then remained rather stable. On the other hand, when the initial potential value was more negative, it was relatively stable for tens of hours (E, = - 1.047 V, Fig. 2). Thus, the initial potential was
AND DISCUSSION
When the electrode came in contact with the potassium surface, no gas appeared (although it would be easily observable under the given conditions). Law et ar.[9] however, observed gas evolution in contact with KAlC14-PC electrolyte. We were unable to observe gas evolution in the course of 12,000 h even during periodic anodic polarization at 3 mAcmm2. Since corrosion of alkali metals in organic electrolytes is doubtless a complex effect of many factors, at least those introduced by experimental technique must be
I 0
I
1
2
3
4
I
I
5
6
I
t [ 10’Hrsl
Fig. 2. Change of potential with time after switching off the polarizing current on potassium electrode immersed in electrolyte prior to polarization for 200 h.
769
Potassium electrode
apparently characteristic for the primary stationary passivating layer. The more negative potential indicated probably a lower contamination of the electrode by the residual gases from the vacuum, an effect which cannot be fully excluded. The open-circuit potential (E,) values in the indicated range, or more positive, were also obtained with electrodes after switching off their anodic polarization. This can be attributed to partial depassivation caused by a transitory disturbance of the primary passivating layer giving rise to a corrosion current, by which the electrode was polarized ancdically (see below). The behaviour of electrodes which were for various time intervals in contact with the electrolyte prior to anodic polarization shows the stability of the primary stationary passivating layer (modified gas-phase passivating layer) under these conditions. When the electrode which was at a potential of - 1.047 V for 200 h was polarized for the first time with acurrent density of 1 mAcmS for 6 h, after switching off the current it attained a somewhat more negative E, (Fig. 2). This could be repeated until the value of E, = - 1.061 V was attained. Subsequent 1 h cathodic polarization at the same current density* showed that the latter value is close to the limit corresponding to a most strongly passivated surface in the given electrolyte when the corrosion current drops almost to zero. Indeed, the value of E, = - 1.060 5 0.001 V has been maintained for 12,000 h (the experiment continues at present). The possibility of a durable attainment of a stable E, value from - 1.039 to - 1.061 V is related to changes in the properties of the passivating layer causing probable changes in the activity of K+ ions at the metal surface. Thus, a 20 mV potential increase may be attributed to a decrease in the activity by a factor of 0.46 (see Appendix). Secondnry stationary
passimting
1
0
5
45
I\._-----------2
-1.05 t
a
20
40
60
80
100 t
120
[secl
Fig. 4. Potential change of potassium electrode with time (section I in Fig. 3). (1) After passing a 0.1 mAh charge: (2) after passing a lOmAh charge, both at an anodic current of 1 mAcm_‘.
-i
l.03
x 9 2 w -1.04 -
-I,05 -
L 0
* We assumed that freshly deposited potassium in contact with the electrolyte is most likely to be passivated.
40 t hnl
Fig. 3. Change of potentia1 with time for a potassium electrode immediately after immersion into electrolyte (see text).
layer
The above results are related to a long-lasting effect of the eIectrolyte prior to the first anodic polarization. The behaviour of the potassium electrode during anoclic polarization after only an hour of contact with the electrolyte is shown in Fig. 3 (curve 1). Its E, was -1.047 V. III the first 100s break between 300s polarization cycles (section I) the course of E, as function of time is shown by curve 1 in Fig. 4. During a 1500 s pause (Fig. 3, section II) the value of E, as a function of time passed reproducibly through a minimum, indicating continued spontaneous depassivation (Fig. 5, curve 1). The rise following after the decrease of the potential caused by a new gradual passivation corresponds apparently to a corrosion current proceeding on open circuit. The described behaviour changed substantially if a charge higher than 10 mAhcmm2 (at 1 mAcm_‘) passed through the electrode. Then, after 64 h idling period E, was close to - 1.058 V and the electrode could be easier depassivated (curve 2 in Fig. 3) or passivated (compare the pauses, Figs 4 and 5, curve 2), the course of the E, value being quite different. The experiments with both groups of electrodes
I
1
Iq 35
IO
500
10X
t [secl
1500
Fig. 5. Potential change of potassium electrode with time (section II in Fig. 3). Same conditions as in Fig. 4.
F. P. DOUSEK AND J. JANSTA
770
suggest that the passivating layer is not stationary, its equilibration being very slow. The corrosion current appearing after anodic dissolution of the first portion of potassium under the passivating layer (Fig. 5, curve 1) is probably related to a structural reorganization of the primary stationary passivating layer, where the passivating products formed primarily on the potassium surface contacting the gas phase influence the electrode potential. This reorganization (or perhaps recrystallization) causes a decrease in the surface coverage with the passivating products and the metal surface set free is covered with the secondary stationary passivating layer of different properties as a result of the corrosion current. A similar effect was observed on a lithium electrode in the medium of thionyl chloride[ lo]. Kinetic passivating
layer
There is no reason to believe that the passivating layer on the potassium electrode must be nonporous. For the disappearance of the corrosion current, it is only necessary that the active potassium surface and the large organic solvent molecules be mutually separated. It seems that the ratio between the free and covered metal surfaces determines the value of the exchange current, which is known to be much lower than expected in the case of such a reactive metal. The surface coverage decreases during anodic polarization (compare E-r curves in Fig. 6). After switching on a constant current, the potential decreases abruptly from E, = - 1.060 V to a value by about 0.7 V less negative, evidence for a strong passivation of the electrode in stationary state. From that moment the structure of the passivating layer starts to change, the electrode is gradually depassivated and its potential shifts to more negative values. The equilibration takes more time the lower is the polarization current: 7-10 h at 1 mAcme2, 80 min at 2mAcme2 and 35 min at 3 mAcm- I. An anodic charge of 1 mAh cm - ’ of electrode surface dissolves a 17 pm thick layer of potassium (nearly 11 monolayers per second), which
shows that the original structure of the primary stationary passivating layer undergoes a substantial change from the very beginning of polarization, and it is notable that the establishment of the kinetic passivating layer structure takes several hours. This may possibly be attributed to recrystallization of the passivating products during intense mass transport. Mechanism
of passivation
Our hypothesis about kinetic changes of the passivation layer is based on the following observations. (1) Relatively high iR-free electrode polarization even after attainment of equilibrium during anodic dissolution shows that a passivating layer always exists on the electrode surface at any current density (the surface coverage is considerable and not constant). At defects in the kinetic passivating layer, where the potassium surface is momentarily free, the actual current density is obviously much higher than that referred to the geometric surface area. (2) The current efi%iency of potassium dissolution is close to 100°~. Even when a 3 mm thick layer of potassium was dissolved, the passivating layer did not grow thicker and no sludge formation was observed. (3) The passivating layer adheres firmly to the potassium surface, suggesting the existence of strong adsorption forces. It is known that dissolution of crystalline metals does not proceed uniformly over the whole surface, but preferentially on certain active sites, whose location changes quickly during the dissolution. It seems that the passivation product maintains its contact with the metal surface even during its dissolution, especially in the situation described below at (4). (4) Matsuda et a!.[161 determined the solvation number of K+ ions in PC as N, = 1.3 (in the case of Li+, N, = 3.2). Hence, K ’ ions transferred from the anode during anodic dissolution take off from the active sites (defects in the kinetic passivating layer) a considerable quantity of the corrosion medium, PC. The higher the polarizing current, the less PC molecules are available for the corrosion reaction
2
t [Hrsl
3
Fig. 6. E-S curves for anodically polarizedpotassium electrode. (1) i = 1 mA cm-‘; (2) i = 2 mA cm-‘;
= 3 mAcm_‘.
(3) i
Potassium electrode (1) at the active sites of the electrode surface (at the bottom of the defects in the kinetic passivating layer). Thus, at a higher polarizing current a higher fraction of the electrode surface can persist in the active state. Scarr[l] arrived at a similar conclusion in the case of anodic polarization of a lithium electrode. When the equilibrium surface coverage with the passivating layer is reached, the true current density on the free portion of the potassium surface may be almost independent of the overall current density, and probably this is the reason why electrodes loaded for an hour with different currents attained almost the same potential (Fig. 6). As soon as the polarizing current is switched off, the transport of PC molecules from the metal surface stops and their concentration is restored by diffusion, the corrosion reaction (1) starts, and the value of E, corresponding to a newly formed secondary stationary passivating layer is immediately established (Fig. 5, curve 2). As in the case of establishing the stationary potential of a polarized electrode, the course of the E, value with time depends on the preceding polarizing current density, ie the degree of disturbance of the starting primary stationary passivating layer, as follows from Table 2. CONCLUSIONS The surface of an even potassium
electrode
most carefully prepared is always covered with a gas-phase
passivating layer due to reaction with atmosphere components. This is supplemented with products of corrosion reactions between the metal and the electrolyte to Form a primary stationary passivating layer. The coverage of the potassium surface with the latter need not be complete but, anyhow, further PC molecules cannot get into contact with remaining active sites on the metal. The equilibrium electrode potential attains values from - I.040 to - 1.06i V against a KHgx electrode, according to the degree of passivation, its less negative values indicating some influence of the gas-phase passivating layer on the primary stationary passivating layer. On a polarized anode surface, the latter changes to a kinetic passivating layer, distinguished by a lower coverage of the metal surface dependent on the current density. If the anodic polarization lasts for a sufficiently long time, the temporarily uncovered parts of the active surface of potassium are blocked by corrosion with the electrolyte immediately after switching ofF the current to form a secondary stationary passivating layer, which is no more influenced by the gas-phase passivating Iayer. The components of all passivating layers adhere Table 2. Influence of preceding anodic polarization on the rate of potential equilibration of potassium electrode in KPF,-PC electrolyte at 25°C 2 h anodic
current
(~AcII-~) 1 2 3
Open-circuit potential (in V) after t h of current interruption Z=l t+0 t = 4x -1.048 - 1.042 - 1.038
- 1.053 - 1.048 - 1.044
- 1.0615 - 1.0605 - 1.0600
771
strongly to the potassium surface (probably by chemisorption) and therefore they do not lose their contact with the metal during its anodic dissolution. This is obviously why the passivating layer thickness does not visibly increase even on passing a charge of 0.18 Ahcmm2. Acknowledgements-The authors are indebted to Dr. K. Micka and Prof. J. Koryta of this Institute for helpful discussions and stimulating interest.
REFERENCES 1, R, F. Starr, J. elecrrorhem SM. 117, 295 (1970). 2. R. Jasinski and S. Carroll, J. electrochem. Sot. 117, 218 (1970). 3. B. Serosati, Electrovhim. Acra 26, 1559 (1981). 4. S. P. S. Yen, D. Shen and R. B. Someano, Meeting of the Electrochemical Society, Hollywood, Florida 1980. Extended Abstracts, p. 99. The Electrochemical Society. Pennington (1980). 5. F. Schwager, Y. Cetonov and R. H. Muller, ibid., p. 102. (1980). 6. E. Peled, J. Power Sources 9, 253 (1983). 7. F. W. Dampier, J. ebrlrochem. Sor.128, 2501 (1981). 8. K. M. Abraham, J. L. Goldman and M. D. Dempsey, J. &clrochem Sot. 128, 2493 (1981). 9. H. H. Law. R. Atanasoski and Ch. W. Tobias, Meeting of the Electrochemical Society, Hollywood, Florida 1980. Extended Abstracts, p. 106. The Electrochemical Society, Pennington (1980). 10. N. A. Fleischer and R. J. Ekern, J. Power Sources 10. 179 (1983). 11. F. P. Dousek, J. Jansta and J. &“a, J. eleczroannl. Chem. 46_ 281 (19731. 12. F. &. Doisek &d J. Jansta, Electrochim. Acta 20, I (1975). 13. J.Jansta, F. P. Dousek and J. &a, J. electrooncd. Chem. 38, 445 (1972). 14. F. P. Dousek, J. Jansta and J. l%%a, Chemick6 Listy (in Czech.) 67, 427 (1973). 15. 1. Jansta, F. P. Dousek and J. iiha, .I. elettroun~1. Ch.zm. 44, 263 (1973). 16. Y. Matsuda, H. Nakashima, M. Morita and Y. Takasu, J. electrochem. Sot. 128, 2552 (1981). 17. M. H. Armbruster and J. L. Crenshaw, J. Am. ckem. Sot. 56, 2525 (1934). 18. R. Jasinski. High Energy Batteries. Plenum Press, New York (1967). 19. A. J. de Bethune. The Encvciooedia of Elec’trochemistrv (Edited by C. A. ‘Hampel).‘R&nhold,“New York (1964j. 20. G. M. Giordano, P. Longhi, T. Mussini and S. Rondinini, J. Chem. Thermodyn. 9.997 (1977). 21. G. N. Lewis and F. G. Keyes, J. Am. them. SDC. 34, 119 (1912).
APPENDIX Calculation of the potenliol of potassium agains& potassium amnigam reference electrode We shall consider the galvanic cell KHg,(l)IKPF,(0.5
M)(PC)]PLIK(s).
(Al)
aZ,K+ aI.K+ %z The electrode on the right-hand side is a reversible electrode of the first kind, where the solid potassium surface is covered with a polymolecukr passivating layer (PL) functioning as a solid electrolyte. We assume that the activity of K+ ions in it, n2,K-, is different from that in the cell electrolyte.0.5 M KPF,
772
F. P. Dousm
in PC, where it is equal to 0, K+. The potential hand electrode can be expre&d as EKtlK = ERtjK
of the right-
RT + ~ In a2 K‘ F ’
where Eg+/K denotes standard potential of potassium electrode in the given medium. The left-hand electrode of the cell is a liquid amalgam electrode of the first kind containing 1.129 atm y0 K in contact with 0.5MKPF, in PC. Such an electrode is not covered by any passivating layer[l l-151. Its potential can be expressed as EKH~,,u*
=
E&,/K
RT ul K* +f In --!-F UK
(A3)
AND
J. JANSTA
diffusion potential, however we have no quantitative idea in this respect. Equation (A4) requires the knowledge of standard potentials in the corresponding media, the difference from those in aqueous solution is given by the difference of solvation energies of K+ ions[lS], however no data in this respect are available. If the passivation layer were nonexistent, then Q[,~+ would beequal to u~,~+ and the last term in Equation (A4) would he zero. Then, we could use the known standard potentials in aqueous medium, since their shift by the salvation energy of K* ions would be eliminated by their difference occurring in Equation (A4). The values in aqueous medium are[19,20] EkejK = -22.925V, This would
where E&s ,K+ denotes its standard potential in the given medium ani c1K is the activity of potassium, which was determined as 0.02419 by linear interpolation of tabulated data[17] in mol fraction units, the (rational) activity coefficient being equal to 2.14. The electrical potential difference E of the cell considered corresponds to two reversible processes, namely amalgamation of potassium and transfer of K’ ions from the passivating layer (activity 02) into the electrolyte (activity a, ), or vice versa. By substracting Equation (A3) from (AZ) we obtain RT
(A4) Here, the first three terms on the right-hand side correspond to the free energy of amalgamalion and the fourth refers to the osmotic work due to a difference in the activity of K+ ions between the passivating layer and the electrolyte. The interface between these two media gives rise to the formation of a
Eo,,,x,K+ = --1.975V
at 25°C.
give
E = -2.925
+ 1.975 + 0.05916 logo.02419
= - 1.0456 v, which agrees very well with the value measured by Lewis and Keyes[21], - 1.0481 V, for a potassium-potassium amalgam (1.12665 atm % K) cell in ethyl amine-K1 electrolyte. (The potential of their amalgam differs as little as 5 x lo- ’ V from ours.) They stated that no reaction between ethyl amine and potassium took place, hence no passivating layer was formed. The equilibrium potential values for the cell (Al) measured by us were in the range from - 1.039 to - I .06O V. The latter value was reproducible to within + 1 mV as the limiting value corresponding apparently to a steady structure of the secondary stationary passivating layer formed in each experiment after a sufficiently long time. The difference relative to the calculated value (- 1.045 V) is obviously due to the influence of the passivating layer (change of the standard potential and the activity of Kf ions in this layer). Thus, a decrease in the activity of K+ ions in the passivating layer by 56% would cause a 15 mV increase of the K electrode potential, which would account for the difference. Structural changes of the passivating layer are apparently accompamed by changes of the equilibrium potential of the potassium electrode (Fig. 2).